linux-kernel - Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf

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Date:   Fri, 18 Aug 2023 14:20:33 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Francis Laniel <flaniel@...ux.microsoft.com>
Cc:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for
 'perf_kprobe' PMU

On Fri, 18 Aug 2023 20:13:43 +0200
Francis Laniel <flaniel@...ux.microsoft.com> wrote:

> Hi.
> 
> Le vendredi 18 août 2023, 17:41:41 CEST Steven Rostedt a écrit :
> > On Fri, 18 Aug 2023 21:37:05 +0900
> > 
> > Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:  
> > > That's why perf probe uses the offset from '_text'. Normal KASLR will just
> > > moves all symbols. (Finer one will move all symbols randomely)
> > > This should not need to access /proc/kallsyms but vmlinux or SystemMap.  
> > 
> > We could just pass in: "_text+offset" too.  
> 
> So, the idea would be to change the existing create_local_trace_kprobe() and 
> above functions to indicate the user's offset is to be used against _text and 
> not address?

No, not to modify that function, but if you know the offset from _text (via
the vmlinux), you can easily calculate it for that function.

I mentioned having a way to pass in the vmlinux debug info address and
subtract the kaslr_offset from it. But that's actually unnecessary. If you
have the address of the function you want, and the address of _text, both
from the debug info of vmlinux, you can simply pass in "_text+offset", and
then use kallsyms to give you _text, and add the offset to give you the
address for create_local_trace_kprobe().

-- Steve